1. Field of the Invention
This invention relates to superlattice quantum cascade lasers.
2. Description of the Related Art
FIG. 1 illustrates the miniband structure of a conventional optical gain medium 10. The optical gain medium 10 includes active layers 12, 12xe2x80x2 and injector layer 14, which is interposed between the active layers 12, 12xe2x80x2. The active and injector layers 12, 12xe2x80x2, 14 have miniband structures in their conduction bands. The miniband structures include upper and lower minibands 16, 18, in the active layers 12, 12xe2x80x2, and transport miniband 20, in the injector layer 14. The miniband structures are produced by semiconductor superlattice structures in the active and injector layers 12, 12xe2x80x2, 14.
The structure of the upper and lower minibands 16, 18 of the active layers 12, 12xe2x80x2 fix the wavelength of the light emitted by gain medium 10. In particular, in quantum cascade (QC) lasers, charge carrier transitions from the upper to lower minibands 16, 18 produce the light emissions. Thus, stimulated optical emissions have wavelengths that corresponding to the energy of the miniband gap 22 separating the upper and lower minibands 16, 18.
Transport miniband 20 of injector layer 14 transports de-excited charge carriers from lower miniband 18 of active layer 12 to upper miniband 16 of adjacent active layer 12xe2x80x2. The transport miniband 20 replenishes inverted populations of charge carrier in the upper miniband 16 of the active layer 12xe2x80x2. Thus, the upper miniband 16 is the target upper miniband of the associated transport miniband 20. Keeping the inverted population of charge carriers replenished enables further optical emissions from the active layer 12xe2x80x2.
Various embodiments of optical gain medium have injector and active layers in which associated transport and target upper minibands have mismatched lower edges during pumping. The mismatches enable relaxation processes to reduce densities in the active layers of charge carriers that have energies of the associated transport minibands. Reducing densities of such charge carriers in the active layers reduces backward transport of charge carriers through the injector layers. Reducing backward transport increases obtainable inverted populations of charge carriers over those obtainable in optical gain media that are similar except that such mismatches between associated transport and target upper minibands do not exist.
In one aspect, the invention features an optical gain medium having first and second active layers and an injector layer interposed between the first and second active layers. The active layers have upper minibands and lower minibands. The injector layer has a miniband that transports charge carriers from the lower miniband of the first active layer to an excited state in the upper miniband of the second active layer in response to application of a voltage across the optical gain medium.
In another aspect, the invention features a process for operating an optical gain medium with a plurality of active layers. The process includes transporting charge carriers from a lower miniband of one of the active layers to an upper miniband of an adjacent one of the active layers and relaxing the transported charge carriers to lower energy states in the same upper miniband.
In another aspect, the invention features an apparatus that includes an optical gain medium and electrical contacts adjacent opposite sides of the medium. The optical gain medium has a series of stages. Each stage includes an injector layer and an adjacent active layer. The active layers have upper and lower minibands that are separated by a miniband gap Emg. The lower minibands have a width xcex94lm. The electrical contacts are able to apply a voltage Vps across each one of the stages. Emg is smaller than qVpsxe2x88x92xcex94lm with q being a charge of carriers in the minibands.